Centrifugal compressors in process gas service generally require rotary shaft sealing to prevent the process gas from escaping the compressor casing in an uncontrolled manner into the atmosphere. Typically, multi-stage “beam” style compressors require two seals, each disposed at an end of the rotary shaft, whereas single-stage, “overhung” style compressors require a single rotary shaft seal disposed directly behind the impeller. In the past, oil film seals were used in many applications to prevent the leakage of the process gas; however, the use of dry gas seals in place of oil film seals has increased dramatically in recent years.
Generally, dry gas seals function as mechanical face seals and include a mating (rotating) ring and a primary (stationary) ring. During operation, grooves in the mating ring generate a fluid-dynamic force causing the primary ring to separate from the mating ring creating a “running gap,” typically 3-10 microns, between the two rings. A sealing gas may be injected into the dry gas seal, thereby providing the working fluid for the running gap of the dry gas seal, which forms a non-contacting seal between the atmosphere or flare system and the internal process gas of the compressor.
The sealing gas injected into the dry gas seal may be process gas fed from the discharge line of the centrifugal compressor. Typically, a portion of the sealing gas injected into the dry gas seal may be vented via a primary vent to the atmosphere or flare system. However, it is often desirable to return the sealing gas to the centrifugal compressor for processing, thereby improving efficiency. The foregoing may be accomplished via the redirection of the sealing gas flow through the primary vent to the suction side of the centrifugal compressor.
Generally, in centrifugal compressors, inboard (i.e., axially inward along the rotary shaft toward the centrifugal compressor) of the dry gas seal is an inner labyrinth seal, which separates the process gas from the dry gas seal, and outboard (i.e., axially outward along the rotary shaft away from the centrifugal compressor) of the dry gas seal is a barrier seal, which is typically provided proximate the end portion of the rotary shaft adjacent the shaft bearings to prevent the lubricant provided to the shaft bearings from contaminating the dry gas seal. A separation gas supply may be injected into the barrier seal to create a barrier to prevent the migration of lubricant, e.g., oil, into contact with the dry gas seal. Typically, an inert gas, such as nitrogen, or air may be utilized as the separation gas. In a conventional seal assembly, as a result of the injection of the inert gas into the barrier seal, a portion of the inert gas may leak across the barrier seal and may be routed through a secondary vent for removal from the centrifugal compressor.
In addition to the inert gas routed through the secondary vent, often a residual portion of the seal gas may bypass the primary vent, thereby being routed to the secondary vent for removal from the centrifugal compressor. Accordingly, a mixture of process gas and inert gas may be discharged from the secondary vent. It would be desirable to recycle this mixture to the suction side of the centrifugal compressor for further processing and improved efficiency. However, the inclusion of the inert gas in the secondary vent discharge may negatively impact the operation of the centrifugal compressor if fed therethrough.
What is needed, then, is a sealing assembly for a centrifugal compressor, such that inert gas from the separation gas supply is prohibited from entering the secondary vent. Such a sealing assembly provided would further be configured to substantially reduce or prevent the uncontrolled leakage of process gas into the atmosphere.